There is an advantage to have drive A external to the ST as it allows to have a single Gotek/HxC to use on multiple STs.
Here are some infos about how to put a switch in the ST which changes the internal to external drive and vice/versahttps://rdemming.home.xs4all.nl/Atari/DriveSwapAtariST/

But it needs cutting YM pins or PCB traces. And today people try to avoid modding too much the old STs. So is there another solution?

This means that selecting A will now switch to both external and internal drives, which obviously is bad.
But as the internal drive is on a ribbon cable, it should be not too difficult to make the internal get the drive1 signal.

Been doing some thinking about this. One (less intrusive/more reversible) solution would be to desolder the YM and add a 4pdt IC (such as ts5v330) and an Attiny on an addon PCB. Then either having the attiny read reset at boot (connected to R100); hold reset at power on to swap A/B or having it connected to the GPO pin on the YM and make a binary to swap A/B by swapping state (can't remember right now if there's a pull-up or pull-down on the ST).

The latter solution would be 100% STE proof since the GPO isn't routed anywhere on the mobo, STs have it routed to pin 5(?) on the video out. But on the other hand; have anyone seen some hardware made to use that pin, ever?